Pressurized dispensing containers having compartments separated by a piston are well known. Product is contained in a first compartment having outlet means such as a valve, with a movable piston separating the first compartment from a second compartment within which a propellant is located. The piston prevents mixing of the propellant and product to avoid contamination or other detrimental effects to the product.
Under pressure of the propellant, the piston forces the product out of the container when the valve is actuated. The piston uses the product as a sealant between the container wall and piston, relying on close tolerances in manufacture to provide a minimum gap to minimize permeation of the propellant into the product. The piston cannot be tight with the container wall since it must be free to move. Typically pistons are rigid, including ridges or lips to wipe along the container sidewall.
Existing pistons experience problems due to the close tolerances required for the system to operate. If the gap is too large, a substantial amount of propellant may leak by the piston. If too small, the piston may bind and prevent product discharge, or buckle and allow the propellant to blow-by the piston. Even if the piston is made to close tolerances, the container diameter is such that it may be out-of-tolerance or the container may be out-of-round. Denting of the container is also a problem which must be considered as such dents may cause the piston to bind. Because of the close tolerances required, there is a fairly high failure rate among containers using conventional piston constructions.